The present invention relates to a vehicular automatic transmission for changing power transmission paths by controlling operation of clutches for automatically shifting gears of the transmission.
Automatic transmissions are arranged to shift gears automatically dependent on running conditions of a motor vehicle to achieve desired vehicle running characteristics. It is customary to provide a gear shift map composed of upshifting and downshifting curves for each gear position, the curves being established in relation to the vehicle speed and the engine power output, and to control the automatic transmission to shift the gears according to the gear shift map dependent on the running conditions as indicated on the gear shift map. One example of such a gear shifting control is disclosed in Japanese Laid-Open Patent Publication No. 61-189354.
For controlling the automatic transmission in the above manner, it has been required to shift the gears smoothly to minimize any shock or delay during operation of the transmission. There have heretofore been proposed various measures to meet such a requirement.
When the accelerator pedal is depressed and the transmission is shifted down (a "power-on/downshift" mode corresponding to a kickdown) or when the accelerator pedal is depressed to increase the vehicle speed and the transmission is shifted up (a "power-on/upshift" mode), since the rotational speed of the engine is increased in response to the depression of the accelerator pedal, the engine may race unless the operation of the transmission is controlled adequately, because of the wrong timing of a gear shift, a reduction in the hydraulic pressure for controlling transmission, and other undesirable conditions. When engine racing occurs, a shock may be developed by the transmission upon a gear shift, and the driver may have a bad feeling during the shifting of transmission gears.
If the occurrence of engine racing and the magnitude thereof can be detected precisely, then such engine racing, when it has occurred, can be suppressed by controlling the hydraulic pressure supplied to the clutches in the transmission or controlling the power output of the engine. However, it has been difficult to accurately detect engine racing. Conventionally, the occurence of engine racing has been detected based on a change in the rotational speed of the engine. With such a conventional detecting process, no accurate detection of engine racing is possible since the rotational speed of the engine is affected by a slippage of a torque converter which is coupled to the output shaft of the engine.